According to the well-known Hall effect, if a magnetic field is applied along a z-axis to a bar that carries a current along an x-axis, an electric field is produced along a y-axis. The electric field is proportional to the magnetic field and current density. The electric field can be sensed and used to determine the magnitude of the magnetic field or at least to determine when there is a significant change in the magnetic field.
A Hall cell is a sensor that utilizes the Hall effect. The Hall cell has a current-carrying bar and leads coupled across the bar for providing a signal that indicates the electric field. Such sensors can be integrated into an integrated circuit (IC) chip and packaged along with other circuitry.
A magnetic field between a magnet and a device made of a ferrous material can vary depending on a distance between the magnet and the device. Consequently, a sensor with one or more Hall cells can be used to sense distance or a change in distance when positioned between the magnet and the device. One known use for such a sensor is as a gear-tooth sensor for sensing a tooth or notch in a moving body; e.g., the body can be a cam shaft of an automobile, in which case the sensor can sense ignition timing. Such a sensor is shown, for example, in U.S. Pat. No. 4,970,463.
To improve sensing, a flux concentrator can be mounted between a sensor and a permanent magnet, as shown in the incorporated patent. However, a manufacturer of the sensor can only recommend that a customer do so if desired. In particular sensing systems, it may be difficult to provide such a flux concentrator in a useful position, and even with such a concentrator, there are gaps between the concentrator and sensing circuitry because of the width of the packaged sensor. Such gaps reduce the efficiency of the concentrator.
It is known that there is a leadframe with a paddle and leads made of alloy 42, a material with a permeability of 4,500 at B=20 gauss to a maximum of 70,000 Gausses/Oersteds. Such a leadframe would be undesirable for use with a magnetic sensor, however, because the leads of the leadframe would tend to divert the magnetic field away from the sensing circuitry.